1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a copier, a printer, a facsimile machine, a multifunction machine including at least two of those functions, and an image quality correction method used therein.
2. Discussion of the Background Art
In general, a multicolor image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine including at least two of those functions, etc., forms multicolor images using multiple different color developers on sheets of recording media.
In addition, there are multicolor image forming apparatus that can switch between a multicolor mode and a monochrome mode.
FIG. 7 schematically illustrates a configuration of an electrophotographic multicolor image forming apparatus.
As shown in FIG. 7, the multicolor image forming apparatus includes four image forming units 100Y, 100C, 100M, and 100Bk each of which forms a different single-color image, for example, yellow, cyan, magenta, and black images electrostatically on a photoreceptor 200. Each of the image forming units 100Y, 100C, 100M, and 100Bk forms an electrostatic latent image on the photoreceptor 220 and then develops the latent image with the developer. The image forming apparatus further includes a transport belt 300 that is looped around multiple rollers such as a driving roller, a driven roller, and a support roller and is rotatable in a direction indicated by an arrow in FIG. 7 to transport a sheet of recording media. Each photoreceptor 200 contacts an outer surface of the transport belt 300, forming a transfer nip therebetween.
In the multicolor mode, the transport belt 300 contacts all four photoreceptors 200. As the transport belt 300 rotates, the different single-color images formed on the respective photoreceptors 200 are transferred therefrom and superimposed one on another on the sheet that is transported by the transport belt 300.
By contrast, in the monochrome mode, one of the rollers around which the transport belt 300 is looped around moves so as to disengage the transport belt 300 from other photoreceptors 200 than the photoreceptor 200 for black as indicated by a dotted line shown in FIG. 7. The purposed of such an operation is to reduce deterioration on and wear of the photoreceptors 200 that are not used in the monochrome mode, insofar as those photoreceptors 200 are not driven in the monochrome mode.
Additionally, multicolor image forming apparatuses generally include a mechanism for correcting image quality because changes in image density and/or relative positions of the respective single-color images can be caused over time, degrading image quality. Such changes are caused because operational conditions of the image forming units and/or environmental conditions change over time.
A typical image quality correction method includes forming a test pattern for image quality correction on each photoreceptor, transferring the test patterns from the respective photoreceptors onto the transport belt, and detecting the test patterns with a detector. Then, image quality is corrected based on image characteristic data obtained from results of the detection.
For example, in image quality correction, a known multicolor image forming apparatus forms an image density patch (graduation pattern) for each of yellow, cyan, magenta, and black that are color components of the multicolor image and then detects the image density patches with an optical sensor. However, this known image forming apparatus corrects image quality of all colors even when use frequency of the monochrome mode is higher than that of the multicolor mode, wasting respective color toners other than black toner.
To save color toners other than black toner, another known image forming apparatus switches between multicolor image quality correction and monochrome image quality correction as required. Image quality of all colors is corrected in multicolor image quality correction while image quality of only black is corrected in monochrome image quality correction. Thus, when black image quality requires correction, an image density patch for only black is formed, thereby saving respective color toners other than black toner.
Still, such a known image forming apparatus poses an inconvenience because the transport belt should be disengaged from those photoreceptors other than the photoreceptor for black when monochrome image quality correction mode is to be executed after the multicolor image forming mode is executed. In particular, when image quality correction is required during an image forming operation, the image forming operation is stopped and then the transport belt is disengaged from the photoreceptors other than the photoreceptor for black in order to correct image quality. The time period required for changing a position of the transport belt means downtime for the image forming apparatus.
In view of the foregoing, there is a need to reduce downtime due to image quality correction as well as to save toner, which known methods fail to do.